1. Field of the Invention
The present invention relates to a memory system, e.g., a memory system that converts write data input to a NAND flash memory into a pseudo random number.
2. Description of the Related Art
Recently, demands for large-capacity nonvolatile semiconductor memories are increasing with the rapid spread of digital cameras, portable audio players, and the like, and NAND flash memories are widely used as the nonvolatile semiconductor memories. Recent micropatterning is increasing the capacities of the NAND flash memories, and this worsens the operation reliability.
As disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 11-339495, therefore, a method of converting write data into a pseudo random number by using a random data generator using the M-sequence (Maximum length code) or the like is adopted. This method disperses data to be written into a block of a NAND flash memory, thereby preventing concentration of data patterns.
In the NAND flash memory, however, an unselected cell is made unselected for data write by using coupling (boost) of a word line and channel. When a certain specific pattern continues, therefore, insufficient boosting may cause a write error if cell threshold levels to be written in a block are biased. In a case like this, it is sometimes impossible to completely correct the error by an error check and correction (ECC) circuit. This makes it difficult to well secure the data reliability.